Probably more difficult/ costly to make and less popular - it's just fallen out of style... the bowed glass also distorts things a bit and most were framed. people like the more streamlined rimless look now
i have both a 72g i got used and 46g bowfront i bought new a few years ago and in setting them up it can be difficult to get things like stands/ lids... and both have a center brace which can be a pain to work around for lighting...